Large-screen Television Vision Technology Content: Types of TV display system Liquid crystals Polarization of light Light modulation through polarization control Opto-electric effect Issues of pixel addressing

TV display Types (Digital Light Processing)

Type of display

Basic information of liquid crystal

Liquid crystals The liquid crystals are one of the most fascinating material systems in nature, having properties of liquids (such as low viscosity and ability to conform to the shape of a container) as well as of a solid crystal. Their ability to modulate light when an electric signal is used has made them invaluable in flat panel display technology. Liquid crystals have anisotropic properties similar to solid crystal because of the ordered way in which some of the constituent molecules are arranged. However, the liquid crystal have low viscosity and can flow. The liquid crystals are essentially a stable phase of matter called the mesophase existing between the solid and the liquid. The crystal is made up of organic molecules which are rod-like in shape with a length of ~ 20A0 - 100A0. A perfectly ordered arrangement of such a molecule can lead to a solid crystal. On the other hand, at high temperatures, a (disordered) liquid state is produced.

Different types of liquid crystals The orientation of the rod like molecule defines the "director" of the liquid crystal. The different arrangements of these rod-like molecules leads to three main categories of liquid crystals: Liquid crystals Smectic Nematic Cholesteric Molecules within a layer are ordered long-range orientation order is present No well defined layer order Long-range orientation order is present Well defined order within layers Long-range "twist" between molecules on each layer

Twisted liquid crystals This type of liquid crystal systems are called twisted nematic and a total rotation of 90 can be produced. If the twist angle is increased to enhance the effect, the film becomes unstable if normal nematic films are used. For example, if a twist of 270 is desired, the stable state is one with a -90 twist. However, if cholesteric liquid crystals are used in which there is already a built-in twist, the 270 twist is possible. Such structures are called supertwisted LC .

Unpolarized light Light is a transverse wave, it can vibrate in a variety of directions compared to its direction of motion. In unpolarized light, the fluctuations in the electric field occur in all directions. It is random. Direction of wave motion Electric field varies in all directions Most of the lights we see are unpolarized

Polarized light Light is a transverse wave, it can be polarized. Longitudinal waves cannot be Polarized. In polarized light, the electric field oscillates in only one direction. Direction of wave motion Field varies in only one direction S D

Polarizers Unpolarized, random light can be made to be polarized with the aid of a type of filter. Unpolarized light Polarizer Polarized light The polarizing filter acts like a gate that allows only one direction of motion.

Pairs of polarizer Intensity adjusting polarizer 1st Polarizing filter Unpolarized light Polarized light 2nd Polarizing filter Polarized light with reduced intensity The end result is polarized light of a particular reduced intensity. Intensity adjusting polarizer With the pair of polarizing filters at a 0 degree angle with each other, a maximum amount of light emerges. With the pair of polarizing filters at a 90 degree angle with each other, a minimum amount of light emerges, virtually 0 intensity. By adjusting the angle between the direction of the two filters, the intensity of the light can be controlled.

Light modulation through polarization control A most useful technique to modulate an optical signal is through the use of polarizers and an active device that can change the polarization of light. The general approach is illustrated in figure below. In this particular geometry (other geometries are also possible) two polarizers aligned in the cross-polarized configuration are placed on each side of the device. The device consists of a crystal (or liquid crystal) in which the two refractive indices nre and nro are different. Also, it is possible to alter the difference between nre and nro by using an external perturbation. This alteration can be done by applying an electric field and utilizing an effect called the electro-optic effect.

Polarization and modulation properties of a twisted nematic An important and accurate approximation that is used to describe how light propagates (i.e., how the polarization changes) through a twisted nematic crystal is called the adiabatic approximation. The adiabatic approximation depends upon the fact that the twist, in the crystal is "slowly varying." This is a good approximation for liquid crystals, since a twist of /2 is produced over several microns (say - 10-20 m). As a result, the light responds according to the local refractive indices and the local polarization axes. Thus, if light enters the crystal along the "slow polarization“ direction, it remains along this polarization as it travels down the liquid crystal. Polarization in a twisted nematic liquid crystal change : i) Due to the phase difference between the two rays. ii) polarization is rotated due to the twist in the crystal.

Distortion in liquid crystal There are three main types of distortions that can be produced in a nematic liquid crystal: Splay, where a force causes the rod-like molecules to distort Twist, which is produced by causing a rotation in the alignment of the molecules Bends, where the crystal is distorted so that a bend is produced in the rod like molecules. The elastic constants defining the energy per unit length to create these distortions are denoted by K1, K2, and K3, respectively. Typical values of these elastic constants are in the range of 10-5 to 10-7 dyne.

Orientation of liquid crystal cell To exploit the ability of the field to alter the optic axis, several possible configurations of the liquid crystal cells can be used. when light is travelling along the optic axis, there is no change in the polarization due to the changes in nro and nre, since for this propagation, the two are equal. when light is propagating in a direction perpendicular to the optical axis the difference in nre and nro can alter the polarization of light as it travels. In particular, the polarization change by 900 if the cell thickness is chosen appropriately. when light is propagating in a crystal whose optic axis is slowly twisting, the polarization follows the twist in the crystal.

Orientation of liquid crystal cell

Threshold voltages need to change optic axis A threshold can be defined above which the torque due to the electric field is large enough to overcome the restoring elastic torque. The threshold voltage is given by Parallel Orientation: Perpendicular Orientation: Twisted Orientation: The threshold voltages discussed above, do not produce an abrupt change in the optic axis from one state to another. The change is non-linear but not entirely abrupt. Also, it must be kept in mind that even in the transparent state, there is considerable absorption in the liquid crystal. Example 9.1 and 9.2 Jasprit Singh

Transmittance of liquid crystal In an ideal liquid crystal, the transmittance of the liquid crystal cell should change abruptly as shown by the broken curve. However, in real crystals this is not the case, since the crystal twist is relieved gradually and the transmittance change is, therefore, soft as shown.

Transmittance of STN liquid crystal A most important effect of the STN display cell is that when a potential is applied the change over from the 270o twist to no twist is very abrupt. As a result the transmittance-voltage curves are also extremely sharp. This allows one to go from the low transmittance state to high transmittance state and vice versa with a very small change in applied voltage. At present, a wide variety of STN crystals are in use with twists ranging from 1800 to 2700. The key attraction of all these structures is the extremely sharp transmittance-voltage curve.

Challenges in scaling to a display screen LCD has potential application in large display where a million of liquid crystal cells are used The important challenges are physics related and processing related For a large matrix array, the key challenge of addressing the individual pixels. The pixel addressing challenge : To be competitive with CRT technology LCD technology has to offer comparable resolution and picture quality. It should offer the capability of color display as well as gray scale display. This requires one to have millions of pixel on the display. Addressing of large number of pixel is the key challenge for flicker-free image to a human eye. All the pixel elements must be addressed and refreshed, say, 30 times a second, to present a continuous image to the eye. In case of liquid crystal, voltage level must be maintained between the two plates enclosing the liquid crystals.

Addressing of pixel in LCD system Brute force approach: Individual pixel addressing When array size increases beyond a few hundred pixels, individual pixel addressing is not possible. Multiplexed/Matrix addressing approach: Place the elements on a matrix grid and address each pixel one by one by applying appropriate voltage sequences to the rows and columns.

Matrix addressing approach Pixel transmittance responds to the difference between row and column voltages. A strobe signal Vs is applied to the rows. Information signal +VD (for OFF) or -VD(for ON) is applied to the columns. Unselected pixels are maintained at a voltage VD.

Static transmission voltage curve of a LC cell The parameter  describes the non-linear nature of the T-V curve has to be very small if a large sized matrix is to be addressed.

Matrix addressing approach The device response is determined by the rms value of the voltage pulse. The rms value of the voltage pulses over a time period T for the OFF and ON states

Matrix addressing approach

Matrix addressing approach VD is very close to the threshold voltage Vth, but as N increases, the value of Vs increases rapidly. The value of Nmax, depends critically on the rate given by P. Obviously, for a large value of Nmax, one needs a small /Vth value. It is quite difficult to increase the value of Nmax, if /Vth has a value of say, 0.1, which is typical of twisted nematic crystals with a 900 twist. However, supertwisted nematics have a very small value of P and it is possible to increase N to approach several hundred.

Example